Gold(I)-Catalysed Tandem Cyclisation of Propargyl Acetals and Vinyl Esters

The results of our previous comparative study of chemoselective gold(I)-catalysed alkene cycloadditions of propargyl substrates demonstrated that propargyl acetals react by different cyclisation pathways from the corresponding esters, and that they also have significantly higher reactivities. To increase understanding of the chemistry of propargyl acetals and to explore the possibilities of generating new compounds through gold(I)-catalysed reactions, a range of reactions of propargyl acetals with vinyl esters have been carried out. A new type of cyclopropyl-cyclopentenyl products, (1,3-dimethoxy-4,5-diphenylcyclopent-2-en-1-yl)-cyclopropyl ester derivatives, was obtained. A plausible mechanism, including sequential [1+2] and [2+3] cycloadditions, is proposed for these highly regio- and stereoselective gold(I)-catalysed reactions. The cyclopentenylation took place stereoselectively, whereas cis/trans mixtures of diastereoisomers were formed in the cyclopropanation step, with the selectivity being controlled by the bulkier vinylic substituent. The tandem reaction allows the construction of polysubstituted and highly functionalised bicyclic compounds.